Download Disturbance is

Important aspects of community structure
• (2) Functional Perspective – based on species
traits
– IDEA: Characterize the traits of species in the community,
rather than the taxonomic identity.
– ADVANTAGES:
• Traits related directly to environmental forces that vary from site to
site
– Disturbance, temperature, pollution, siltation, etc.
• Traits can be applied across biogeographic boundaries because
they do not depend on species identities, rather functional
characteristics
– DISADVANTAGES:
• Being able to describe species in terms of traits that are “sensitive”
or directly responsive to environmental factors
Some traits for macroinvertebrates:
Europe:
Table 10.2
North America:
Poff et al. (2006, J. North
American Benthological Society)
Some traits for fish:
http://www.cnr.vt.edu/fisheries/fishtraits/
Habitat Template Model
• Species traits should vary along environmental gradients
• Traits that “match” environmental conditions are “selected” for.
Townsend & Hildrew (1994)
Southwood (1977)
• Test of Habitat Template
• Test #1 - Inverts: [Scarsbrook and Townsend 1993]
• 2 streams in New Zealand
• #1 stable, high habitat heterogeneity
• #2 flashy, low habitat heterogeneity
Sediment
Hydrol. Regime
Disturbance Freq.
Richness
Dominance by mobile
mayfly (similar to Baetis)
Drift rate
Sedentary taxa
CPOM/FPOM
Max. size of mayfly
Stream #1
Fine to coarse
"stable"
L
H
Stream #2
Fine gravel/sand
"flashy"
H
L
L
L
H
H
H
H
H
L
L
L
• Test of Habitat Template
• Test #1 - Inverts: [Scarsbrook and Townsend 1993]
• 2 streams in New Zealand
• #1 stable, high habitat heterogeneity
• #2 flashy, low habitat heterogeneity
Sediment
Hydrol. Regime
Disturbance Freq.
Richness
Dominance by mobile
mayfly (similar to Baetis)
Drift rate
Sedentary taxa
CPOM/FPOM
Max. size of mayfly
Stream #1
Fine to coarse
"stable"
L
H
Stream #2
Fine gravel/sand
"flashy"
H
L
L
L
H
H
H
H
H
L
L
L
• Test #2 - Inverts: [Richards et al. 1997]
• Streams in Great Lakes drainage that differ geologically and thus in
“flashiness”
• Test #2 - Inverts: [Richards et al. 1997]
Index of flood intensity
Clinger taxa
Obligate depositional taxa
merovoltine taxa
• Estimated probability of moderate and high representation of different
species traits:
• Long-lived (merovoltine) species rare in flashy flashier stream sites.
• Obligate depositional taxa incrase with pool habitats.
• “Clinger” species (e.g., mayflies) decline with fine sediment.
• Test #3 - Fish: [Poff & Allan 1995]
• Streams in Wisconsin and Minnesota that differ
in terms of hydrologic variability
• Ho: Fish communities will vary in species trait
composition depending on stream hydrology.
2) Define a “gradient” of
hydrologic variability
among the 34 streams
3) Test Ho. (Community types fall out along gradient of
stream stability.)
1) Define 2 different
community “types”
based solely on
species traits
Flashier stream types have fish traits: generalist
foragers, generalist habitat, slow water, tolerant of silt
Models of Community Structure
• Predictor variable(s):
– habitat structure (spatial heterogeneity)
– habitat dynamics (temporal heterogeneity /
disturbance)
– biotic interactions
• Response variables (”community structure")
– species identities (diversity, richness)
– species traits
How do biotic and abiotic factors vary
in space and time?
Basic ecological theory - 1
Habitat Template Concept (Southwood 1977)
- species require different attributes (traits)
depending on habitat stability (disturbance).
(Fig. 3)
merovoltine
taxa
Examples that we’ve seen:
Index of flood intensity
How do biotic and abiotic factors vary in space and time?
Basic ecological theory - 2
– Maximum species diversity at
intermediate level of natural
disturbance.
Diversity or Richness
Intermediate Disturbance Hypothesis (Connell 1978)
- species diversity (or richness) varies with disturbance (or
habitat stability)
• Prediction?
• Mechanism? (What explains pattern?)
– What “kinds” of species will occur at either end of
the disturbance gradient?
• Differences in competitive ability, tolerance, etc.
• “r-selected” and “K-selected” species
– Evidence for IDH in streams?
disturbance
Test of IDH
– Townsend et al. (1997)
• Sampled invertebrate communities in many sites
throughout a river catchment in New Zealand.
• Sites differed in intensity of bed movement
How do biotic and abiotic factors vary in space and time?
Basic ecological theory - 3
Patch Dynamics Model (Townsend, 1989, J. No. Amer. Benth. Soc.)
- combines Habitat Template and Intermediate Disturbance
Basic Premises:
(1)Stream is a patchwork of habitat with different disturbance histories
(2)Stream organisms are very mobile and colonize quickly
 adjacent patches may have very different fauna
 over many patches, there’s a kind of mosaic equilibrium with a
“predictable” assemblage (at the scale of say, a riffle)
(3) Community structure reflects an interplay of biotic and abiotic factors,
depending on the degree of spatial and temporal heterogeneity of the system.
 Temporal heterogeneity (disturbance) sets population sizes and
regulates competition
Spatial heterogeneity (habitat structure) provides refugia, and sets habitat
heterogeneity for niche segregation
(4) Higher diversity possible when
 disturbance intermediate
 habitat heterogeneous
•
•
•
•
•
Habitat comprised of two axes - spatial and temporal heterogeneity
Niche control
–
Competition dominates
(community response to removal
of Glossosoma by parasite
Cougourdella ? (Kohler & Wiley
1997)
Dominance control
–
Competitive replacement
sequence following disturbance
“reset” (Hydropsyche and
Simulium - Hemphill & Cooper
1983)
Founder control
–
Priority effect (Cladophora and
Leucotrichia - Hart 1992)
Mobility control
–
Weedy species (habitat template
studies, e.g., Scarsbrook &
Townsend 1993; Richards et al.
1997)
Phased succession
–
All patches undergo similar
successional sequence due to
synchronized disturbance in
homogeneous environment
How do streams differ in their disturbance regimes?
diversity
Where will we expect to see strong biotic interactions?
disturbance
Some issues about disturbance
• Disturbance is an event that causes mortality(or
alters resources)
• Mortality depends on :
– Habitat characteristics (Refugia)
• Channel morphology / Substrate (mobility)
– Taxonomic group:
• Algae / Inverts / Fish / Riparian vegetation
– Traits of species
• Ability to avoid, for example
– Magnitude of event
- "little" vs. "lots" of mortality
• so … Disturbance is
–  scale-dependent !
–  context-dependent! (different in different settings)
• Important distinction:
– An individual disturbance is an event;
– temporal pattern of events is regime
• Event
vs.
Regime
• (ecological response)
(evolutionary adjustment)
• [some mortality]
["adaptation"]
Q
Q
thresh
1
2
3
4
4
5
Year
6
7
8
9
Some Questions:
• Question #1: If disturbance causes mortality, and
streams highly disturbed, how do species persist??!!
– "Weedy" traits (Fig. 5)
• High mobility (colonizing)
• Fast (or asynchronous) life cycles
• High fecundity
– Specific behavior / life history
adaptations to avoid disturbance
• Spatial refuge:
– Move to spatial refugia (e.g.,
Matthaei et al. 2000)
– Abetus (water bug) and flash
floods
10 days
• Temporal refuge:
– Fast development in flashy system
(e.g., Gray 1981, Fig. 4)
– Time emergence prior to
predictable floods (Lytle 2002)
Low chance for 50 day development
• Question #2: If disturbance causes mortality, but species
avoid disturbance, then is the event really a disturbance? Or
are species well “adapted” ?
• Example: annual predictable disturbances in Rocky Mountain
streams.
Q
Q
Adaptation or
Disturbance?
thresh
1
2
3
4
4
5
6
7
8
9
Year
– Under what conditions can "adaptation" occur?
• Signal is frequent (relative to life span)
• Signal not TOO extreme (survivors needed!)
• Signal-to-noise ratio is high (periodic, predictable)
– What if disturbance is frequent, yet unpredictable?
– Fast development
– Asynchronous, multivoltinism (bet hedging)
Natural Flow Regime and Evolution
Extreme events (floods and droughts) exert primary selective pressure
for adaptation, because they often represent sources of mortality.
(Lytle & Poff. 2004. Adaptations to natural flow regimes. Trends in Ecology &
Evolution)
Types of adaptations: Life History, Behavior, Morphology
• Components of
Disturbance REGIME
•
•
•
•
•
•
Magnitude
Frequency
Duration
Timing
Predictability
Rate of change in flow
• Streams can be
characterized in these terms.
– Poff & Ward (1989, 1990), Poff
(1996) – Figs. 11.6, 11.7 text)
predictable
unpredictable
Intermittent
stable GW
Perennial
predictable
snowmelt
Classification of streamflow
“types” (806 unregulated
streams in US)
flashy
predictable
winter rain
Allows a priori expectations about structuring processes in
communities.
Augusta
Creek, MI
> 90 d per
year no flow
Perennial with high flood
frequency, low flood
predictability, and high dayto-day variation in Q
Perennial with low flood
frequency, low flood
predictability, and low dayto-day variation in Q
Perennial with low flood
frequency, high flood
predictability, and low dayto-day variation in Q
Fig. 10.6 in text